Automatic frequency control system



Aug. 17, 1954 J. L. LAWSON 2,683,377

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed March 27, 1946 2 Sheets-Sheet 1T0 LOCAL g OSCILLATOR REPELLER FIG. I

INVEN TOR. JAMES L. LAWSON BY W ATTORNEY Aug. 17, 1954 Filed March 27,1946 VOLTS VOLTS J. L. LAWSON 2,686,877

' AUTOMATIC FREQUENCY CONTROL SYSTEM 2 Sheets-Sheet 2 OUT-PUT OF 0100:40

I INPUI {n I I OUTPUT OF moo: 40

I I I i I \I I \I INPUT TO CONTROL I saw 0F TUBE 46 o 12 I 1 A 1 VOLTS70 I I I I I5 I V UTPJT OF DIODE 36 UTPUT OF DIODE 36 I I I I I I I I LTIME-- TlME PRF FIG. 2 FIG. 3

I ILfUTPUT OF mom: 40

I FIG.4 I

OUTPUT OF DIODE 36 INPUT TO CONTROL GRID OF TUBE 46 INVENTOR. JAMES L.LAWSON TIME ATTORNEY cillator tubes.

Patented Aug. 17, 1954 UNITED STATES serum OFFICE AUTOMATIC FREQUENCYCONTROL SYSTEM James L. Lawson, Schenectad by mesne assignments, to thesented by the Secretary of the America as repr Navy Application March27, 10 Claims.

This invention relates control systems and mor acting system of frequencfrom ripple.

to automatic frequency e particularly to a fast y control essentiallyfree y adjusting Two types of AFC systems th feed the beat frequency therepeller voltage. are in common use. B

When the local oscillator is follo AFC must follow rapidly to preventeven momentary loss of signals.

Another object peller control V0 is to provide a ripple free reltage forvelocity modulated os- Another object is to provide animproved and y, N.Y., assignor, e United States of 1946, Serial No. 657,384 (0]. 250-36)plateof tube H of fast acting automatic frequency jects and features ofthis invention will become apparent upon consideration of the followingdetailed description when taken together With the accompanying drawings,of which: -l

Fig. 1 is a schematic circuit diagram of an AFC circuit embodying thisinvention; Fig. 2 illustrates the frequency discriminator output voltagewhen the local oscillator is on frequency;

Fig. 3 illustrates the frequency discriminator output voltage when thelocal oscillator is off frequency by a slight amount; and

Fig. 4 illustrates the frequency discriminator output voltage when thelocal oscillator is off frequency by a considerable amount.

In Fig.

plifiers ii and I2. Tube ll receive plate voltage through resistor I Bfrom a source of voltage connected to terminal 17. The screen grid oftube II is also connected to terminal ii. The suppressor grid of tube His tied to its which is grounded through the parall nation of resistor18 and capacitor IS. The I. F. input signals from terminal iii areinjected at the control grid of tube H. Capacitor 20 and variableinductor d in series to the and grounded through capacitor 22 comprisethe output circuit of tube I i, which is tuned to a predeterminedfrequency above the I. F. frequency.

Tube i2 receives plate voltage through resistor equency below theI. F.mount that the output e the I. F. frequency.

8 connects to the junction of capacitor 20 and inductor 2!. The plate ofdiode 36 is grounded through capacitor iii and also connects to terminal33 through resistor 39. The plate of diode All connects to the junctionof capacitor 39 and inductor iii. The cathode of diode 40' is groundedthrough capacitor M and also connects to terminal 33 through resistor42. It will be noted that diode 33 is con.- nected as a negative outputdetector of the output of amplifier l l and that diode it is connectedas a positive output detector of the output of amplifier l2. Thecombined outputs of diodes 36 and Gil appear at termina1 3.8. Thedescribed circuit between I. F. input. terminal iii and terminal 38constitutes, therefore, an I. F. discriminator. The discriminator haszero output when the I. F. input at terminal it is at the designedfrequency. For this condition, amplifiers ii and 12 have equal outputsand detectors 3% and it likewise have equal outputs which are opposed inpolarity, the resultant output at terminal 38 being zero. When the inputfrequency at terminal l exceeds the I. F. frequency, the output ofamplifier H is greater than the output of amplifier i2 and the resultantdetected output at terminal 38 is negative in polarity. When the inputfrequency at terminal it is less than the I. F. frequency, the output ofamplifier l l is less than the output of amplifier l2 and the resultantoutput at terminal 33 is of positive polarity.

The discriminator output at terminal 38 is applied to the control gridof pentode it. Pentode 46 functions dually as an. amplifier and inverterof the discriminator output voltage for correcting the repeller voltageof the local oscillator and as a sweep tube for causing the localoscillator to search when the radar transmitter and local oscillatorbeat frequency pulls so far away from the I. F. frequency that there isno discriminator output. A source of power for pentode it is connectedbetween terminals it and 48. The plate of pentode it is connected toterminal ll through plate load resistor 19. The cathode of tube 16 isconnected to one end of potentiometer 50 whose other end has noconnection but whose slider arm connects to terminal 48. Thediscriminator diodes 36 and 4d are biased to operate at the voltagelevel of tube 46 by connecting terminal 48 to the junction of inductor2i and capacitor 22 andv the junction of inductor 3i and capacitor 32through resistors 5| and 52 respectively. Resistor 5i and capacitor 22and resistor 52 and capacitor 32 provide decoupling networks for thepower supply at terminal 48. The screen grid potential of t be 48 istapped from the junction of resistors 56 and 51 connected in seriesbetween terminal 41 and the cathode of tube 46. The suppressor grid oftube 46 is coupled to the screen grid by capacitor 58. and is tied toterminal 48 through resistor 59. Capacitor 60 couples the plate of tube46 to the control grid. The output from the plate of tube 46 is coupledthrough capacitor 6i and resistor 62 in parallel. to the localoscillator repeller from. terminal 63. The quiescent voltage, for thelocal oscillator repeller is. taken from terminal. 63 through resistor6.4.connecting to the slider arm of potentiometer 65., whose resistorstrip is connectedinseries withresistor 66. between terminals 41 and 48to provide a voltage tapping. arrangement.

When a small negative bias. exists between the control grid and cathodeof tube 48, tube 4% actsas a direct coupled amplifier. However, if thegrid is less negative than the critical amount, tube 46 will act as. a.conventional transitron oscillator because of the screen grid and suppressor grid coupling through capacitor 58. A sawtooth voltage wave thenappears on the plate of tube 46 and, therefore, at terminal 63, causingthe local oscillator frequency to sweep. The repetition rate of thesawtooth voltage produced by tube 45 as a transitron is controlled bythe time constant of capacitortil and resistor 59 in the suppressor gridcircuit. The value of plate load resistor 49 chiefly determines theamplitude of the sawtooth voltage which is sufficient to sweep the localoscillator through a single mode. The mode. in which the localoscillator operates is selected by the value of quiescent repellervoltage tapped off at terminal 63 from potentiometer B5.

In Figs. 2, 3 and 4, three conditions of operation of the circuits ofFig. l have been considered. The local oscillator is assumed to be tunedto a frequency above the transmitter for proper operation. In Fig. 2,the beat frequency between the radar transmitter and local oscillator isexactly the I. F. frequency. The dotted. wave forms It and H representthe voltage outputs of diodes 3G and Mi respectively versus time.appearing at terminal 38. The time over which one voltage sweep takesplace is equal to the pe riod or the reciprocal of the pulse repetitionfrequency of transmitter operation. Heavy line 12 is the resultant ofoutputs is and H, or the discriminator input to the control grid of tube46. For this condition, voltage i2 is zero and the. quiescent voltage atterminal 83 which is applied, to the local oscillator repeller isunchanged. Actually, to assure discriminator stability it would beadjusted to have a slightly negative output when the beat frequencyequals the intermediate frequency. Under this condition, the. controlgrid-to-cathode bias of tube 46 can be set just sufficiently negative byadjustment of potentiometer 50 so that transitron action is precluded.

In Fig. 3, the local oscillator frequency is assumed to be slightly highso that the beat frequency is correspondingly higher than the I. F.frequency. The negative output 13 of detector 36 is now increased andthe positive output 14 of detector 40 is decreased. The resulantdiscriminator input voltage 15 to tube 46 is negative and may have asmall ripple 16,, as shown, depending upon the. discharge time constantsin the. outputs of diode 3.6 and 4t and the transmitter pulse repetitionfrequency. Negative signal. '55 applied to the controlv grid of tube 46results in a positive swing of the plate potential which increases the.voltage applied to the local oscillator; repeller from terminal 63. As.the repeller voltage rises, the local. oscillator frequency drops andvthe beat frequency is restored towards the I. F. f equency.

In Fig. 4, the local oscillator frequency is as.-

sumed to be so high and off frequency that. the. to which tube.

beat frequency is at the frequency- I lis tuned. There is then zerooutput from de: tector 46 as shown by line 11 and maximum negative ouput18 from. detector at which appears at the control grid of. tube. 4.6..The correction proc ess forthe local oscillator repeller voltage is thesame as for the case of Fig. 3.

Should the local oscillator be so far off frequency that no.discriminator output. appears atterminal 38, the control grid of tube 48tends to rise to the potential of terminal 43. Tube 46 then acts. as. atransitron oscillator and sweeps the voltage at terminal 63 through theoperative mode of the local oscillator. As soon as the disvidingconnected, means for combining the criminator develops a negative errorvoltage at terminal 38, the sweep of tube 46 as a transitron ends andtube 46 again becomes a high gain amplifier. Similarly, if the localoscillator frequency is low so that the discriminator provides apositive error voltage at terminal 38, tube 46 willprovide a sweepoutput to correct the local oscillator frequency. Of course, thecathodebias established by potentiometer 50 will determine how much of apositive signal control grid of tube tron instead of as an amplifier.

Thusthe illustrative embodiment of this invention is an AFC system whichproduces a substantially ripple-free control voltage for reflexvelocity-modulated tube oscillators. there is shown and described onlyspecific embodiment of modifications possible thereof will be readilyapparent to those skilled in the art. Therefore, this invention is notto be limited except insofar as isnecessitated by the and scope of theappended claims.

What is claimed is: r

1. An automatic frequency control system comprising, means for applyinginput signals to a first signal channel, said first signal channel proacertain the difference of said input signals from a first predeterminedfrequency, means for applying said input signals to a second signalchannel, said second signal channel providing positive output signalsproportional to the difference of said input signals from a secondpredetermined frequency, means for combining said negative and saidpositive signals, and means responsive to said combined signals forrestoring said input signals to a mid frequency between said first andsecond predetermined frequencies, said last-mentioned means beingoperative in the absence of said combined signals for varying said inputsignal frequency until said combined output signal appears.

2. An automatic frequency control system comprising, means for applyingthe beat frequency signals from a transmitter and local oscillator to afirst tuned frequency signal channel, means for applyin tuned frequencysignal channel, said first and second channels being tuned respectivelyto frequencies an equal amount above and below a predeterminedfrequency, a first means for detecting the output of said first channel,a second means for detecting the output of said second channel, saidsecond detecting means providing an output signal opposed in polarity tothe output of said first detecting means, means for combining said firstand second detected signals, and transitron oscillator means responsiveto said combined signals for correcting the local oscillator frequency,whereby said beat frequency is maintained at said predeterminedfrequency.

3. An automatic frequency control system comprising, an input terminaladapted to receive beat frequency signals from a transmitter and localoscillator, first and second amplifier.

means coupled to said input terminal, said first and second amplifiermeans being tuned respectively an equal amount above and below apredeterminedfrequency, first and second rectifying means for detectingthe outputs of said first and second amplifier means respectively, saidfirst and second rectifying means being oppositely outputs of can appearat the 46 before it acts as a transi- Although" prior art and the spiritnegative output signals proportional to said beat signals to a second ithe invention, the many across the output of said first diode connectedin series with a second capacitor across the output of said secondamplifier means, the cathode of said second diode being adjacent saidsecond capacitor and a second resistor connected between said cathodeand said combining means.

5. A discriminator comprising, an input terminal adapted to receive beatfrequency signals from atransmitter and a local oscillator, first andsecond tuned circuitstuned respectively an equal amount above and belowa predetermined frequency, first and second means for detectingrespectively the outputs of said first and second tuned circuits, andmeans combining the outputs of said first and second detecting means,said first detecting means comprising a first diode, a

first capacitor and a first resistor, said first diode being connectedin series with said first capacitor tuned circuit, the plate of saidfirst diode being connected to said first capacitor, said first resistorbeing connected between said plate of said first diode and saidcombining means, said second detecting means comprising a second diode,a second capacitor, and a second resistor, said second diode beingconnected in series with said, second capacitor across the output ofsaid second tuned circuits,

the cathode of said second diode being connected to said secondcapacitor, said second resistor being connected between said cathode ofsaid second diode and said combining means.

6. Apparatus as in claim 5 wherein said first and second tuned circuitsare first and second tuned amplifying means tuned respectively an equalamount above and below a predetermined frequency.

7. Apparatus of claim 5 and means responsive to said combined outputscontrolling the local oscillator frequency whereby said beat frequencyis maintained at said predetermined frequency, said means beingoperative in the absence of an output from said discriminator to sweepthe frequency of said local oscillator.

8. In an automatic frequency control system comprising a discriminatorand means for applyter and local oscillator to the input of saiddiscriminator, apparatus responsive to the output of said discriminatorfor controlling the frequency of said local oscillator, comprising, anelectron tube having at least a control grid, a screen grid, asuppressor grid, a cathode and an anode, a source of supply potential,an anode load impedance, means returning the more positive terminal ofsaid supply potential to said anode through said anode load impedance,means coupling the output of said discriminator to said control grid,means coupling said suppressor grid to said screen grid, means couplingsaid control grid to said anode, means maintaining said screen grid andsaid cathode at predetermined potential levels, and means coupled tosaid anode means, and wherein said second detecting means comprises asecond of said second diode for abstracting a signaldorcontrollingdhefrequency of said local: oscillatorwhereby said beatfrequency is maintained at a predetermined -fre-, quency, said signalabstracted from ;,said plate being periodic in the absence'oi-an outputsignal from said discriminator-whereby said local oscillatorfrequency isperiodically varied until a discriminator output isproduced.

9 An automatic frequency controlisystem comprising, a discriminator toreceive beat frequency signals from a transmitter and a localoscillator,a transitron oscillatorcoupled to the output oi said discriminator, andmeans: operatively associated with said. transitron oscillator and-,said,discriminator prec1ud-.

ing the oscillator action ofsaid transitron oscillator in the presenceof a discriminator output signal, said transitron oscillator;functioning as an oscillator in the absence of adiscriminator outputsignal, said transitron oscillator functioning as an amplifier in thepresence of a discriminator output. signal, the. output of saidoscillator controlling the frequency of said local oscillator.

10. An automatic ,frequency control system comprising, an. inputterminal-adapted to receive beat frequency signals from a transmitterand a local oscillatonfirst and second tuned amplifying channels tunedrespectively an equal amount above and below aipredetermined frequency,first and second means detecting respectively the outputs of said firstand secondchannels, means combining the outputs of said firstand seconddetecting meanssaid first detecting means comprising a first diode, afirst capacitor, and a first resistor said first diode being connectedin series with said first capacitor across themutput of said firstchannel, the plate of said first diode being connectedv to said firstcapacitor, said first resistor ,being connected between saidplate ofsaidfirst diode and said combiningmeans, said second detecting meanscomprising aseconddiode, a second capacitor, and a second resistor, saidsecond, diode being connected in series with said second capacitoracross the output of said second channel,

having an; input adapted th cat od of said second diode bein connect dtosaid second capacitor, said second resistor Joeins connected betweensaid cathode of said second diode and said combining means, a multigridelectron tube having at :least a cathode, a control grid, a suppressorgrid, a screenvgrid, and an anode, a source of supplycpotential,anyanode load impedance, means returning themore posi-' tire-terminal ofsaid source of supply potential to said anode of said rnulti-gridtube'through said anode load impedance, means coupling said control gridto said combining meanswhereby the combined outputs of ,said first andsecond'detectors are coupled to said control grid, ,rneans coupling saidsuppressor grid to saidscreen grid, means couplingsaid-control gridtosaid anode of saidmulti-grid r tube, means maintaining .said screengrid and-said cathode of saidmulti-grid tube at predetermined potentiallevels, said multiegrid tube, acting as a directiooupled amplifier atcontrol; grid to cathode; potentials at i :said multi-grid tube morepositive than a predetermined: critical negative level, said multi-gridtube acting as a transitron oscillator at control grid tocathodepotentials at said multi-grid tubeless negative than, said criticallevel, means biasingsaid-first and second diodes to the operatingvoltage levels of saidmulti-grid tube, whereby said multi-grid tubefunctions as a transitron oscillator in the'absence of a signal at saidcontrol'grid and said multi-grid tube functions as a direct coupledamplifier in the presence of a signal at said control-grid, andmeans-abstracting a control signal from the anode ofsaid multi-grid tubefor controlling the vfrequency of said local oscillator.

- References Cited inithe tfile of this patent UNITED STATES PATENTSNumber I Name Date 2,304,377 Roberts Dec. 8, 1942 2,357,984 Travis Sept.12, 1944 2,404,568 Dow July 23, 1946 2,434,294 Ginzton Jan. 13, 1948

